The present invention relates to heating systems for directing energy to coatings applied to a substrate. In particular, the invention relates a drying system for drying or curing wet coatings such as printing inks, paint, sealants, etc. in which the coatings are applied to selected areas on the substrate and a conductor conducts heat away from non-selected areas on the substrate while the substrate is heated to prevent overheating of the non-selected areas.
Coatings, such as printing inks, are commonly applied to substrates such as paper, foil or polymers. Because the coatings often are applied in a liquid form to the substrate, the coating must be dried while on the substrate. Drying the liquid coatings is typically performed by either liquid vaporization or radiation-induced polymerization depending upon the characteristics of the coating applied to the substrate.
Water or solvent based coatings are typically dried using liquid vaporization. Drying the wet water-based or solvent-based coatings on the substrate requires converting the base of the coating, either a water or a solvent, into a vapor and removing the vapor latent air from the area adjacent the substrate. For the base within the coatings to be converted to a vapor state, the coatings must absorb energy. The rate at which the state change occurs and hence the speed at which the coating is dried upon the substrate depends on the pressure and rate at which energy can be absorbed by the coating. Because it is generally impractical to increase drying speeds by decreasing pressure, increasing the drying speed requires that the rate at which energy is absorbed by the coating be increased.
Liquid vaporization dryers typically use convection, radiation or a combination of the two to apply energy to the coating and the substrate to dry the coating on the substrate. With convection heating, the air is heated to a desired temperature and blown onto the coating and the substrate. The amount of heat transferred to the substrate and coating is dependent upon both the velocity and angle of the air being blown onto the substrate and the temperature difference between the air and substrate. At a higher velocity and a more perpendicular angle of attack, the air blown onto the substrate will transfer a greater amount of heat to the substrate. Moreover, the amount of heat transferred to the substrate will also increase as the temperature difference between the air and the substrate increases. However, once the substrate obtains a temperature equal to that of the temperature of the air, heat transfer terminates. In other words, the substrate will not get hotter than the air. Thus, the temperature of the air being heated can be limited to a level that is safe for the substrate. Although controllable, convection heating is thermally inefficient because air as well as nitrogen have very low heat capacities. Consequently, high volumes of flow are required to transfer heat.
Radiation heating occurs when two objects at different temperatures are in sight or view of one another. In contrast to convection heating, radiation heating transfers heat by electromagnetic waves. The quantity of energy emitted and the wavelength of the emission are both dependent upon the absolute temperature of the source. As the temperature of the source increases, heat transfer increases exponentially to the fourth power. Increasing source temperature results in shorter wavelengths, while decreasing source temperature results in longer wavelengths. Because radiation heating does not require a medium such as air to transfer heat, radiation heating is fast, powerful and efficient.
However, in contrast to convection heating, the rate of heat transfer is dependent upon the absorptivity of the target, i.e., the rate at which the substrate and coating absorb the electromagnetic waves emitted by the source of radiation. As a result, radiation heating often results in uneven, non-uniform heating of the substrate. Portions of the substrate having greater absorptivity will receive energy and heat up at a faster rate than portions of the substrate having lower absorptivity. For example, dark and rough areas on the substrate have higher rates of absorptivity than light colored or generally reflective, shiny portions. Thus, the dark and rough areas will absorb heat at a higher rate. When radiation is directed at the substrate at high intensities in order to dry or cure the coating more quickly, areas on the substrate which are dry or those areas which are dark or rough may burn or discolor. At the same time, other portions of the substrate which are light colored or smooth may still require additional heat to cure or dry the coating. Although efficient, radiation heating results in either excessive heating where portions of the substrate may be burned or discolored or underheating where the wet coating is not sufficiently dried or cured, resulting in offsetting of the wet coating on adjacent surfaces which come in contact with the wet coating.
Most liquid vaporization dryer systems employ both convection heating and radiation heating. However, most dryer systems also suffer from the inefficiency of convection heating and the non-uniform heating of radiation heating.
Because liquid vaporization is a "hot process" which requires heating of both the substrate and the coatings applied to the substrate to evaporate the base of the coating, the substrate exiting the dryer is often at an elevated temperature of around 240.degree. F. Where the substrate comprises a web of material, the elevated temperature of the web presents a dangerous situation. As a result, typical liquid vaporization drying systems cool the web of the substrate after drying by blowing cool air over the substrate or by running the web of substrate through a series of chill rolls containing liquid coolant. The chill rolls are typically metallic cylinders which contact the substrate and which cool the substrate down to safe temperature levels.
Because radiation-induced polymerization does not require vaporization of the base of the coating, ultraviolet radiation employed by radiation-induced polymerization does not heat the substrate or its coatings. Consequently, radiation induced polymerization is generally considered a "cool process".
In contrast to liquid vaporization, radiation-induced polymerization is only generally suitable for specialized coatings. These coatings typically are composed of varnishes, waxes, dryers and other additives that carry the ink colorant (pigment), control the flow of the ink or varnish on the press and, after drying, bind the pigment to the substrate. The dryers include oligomers, which are partially polymerized molecules such as epoxy acrylates or acrylic esters that are blended with a reactive acrylate diluent. The dryers further include a photoinitiator to catalyze the reaction. During radiation-induced polymerization, ultraviolet or electron beam radiation causes polymerization or cross-linking within the liquid coatings. In other words, the liquid coating is polymerized to turn from a liquid to a solid. Little or no vapor is released by this process. In contrast to liquid vaporization, polymerization is virtually instantaneous. However, the high cost of oligomers and the reactive acrylate diluent restricts the use of such coatings and radiation-induced polymerization to specialty products.